Planetary scientists use crater counts to estimate the ages of planetary surfaces. Which option correctly represents crater counting's purpose?

• A. The atmospheric composition.
• B. Ages of planetary surfaces.
• C. Magnetic field strength.
• D. Orbital periods of moons.

Answer: (B)

Cratering tells you how long a surface has been exposed to space. Each impact leaves a crater, and over time more and more craters accumulate on a surface that hasn’t been resurfaced. By measuring how many craters there are and how big they are, scientists compare that density to a known production rate of craters (calibrated from the Moon and other bodies) to estimate how long the surface has been continuously exposed. This approach gives relative ages and, with proper calibration, absolute ages as well. Remember that resurfacing events like volcanism or erosion can erase craters, which is why crater counts must be interpreted with those processes in mind. The other options don’t fit because crater counting isn’t used to infer atmospheric composition, magnetic field strength, or orbital periods of moons; its purpose is to gauge how old a planetary surface is.